The best Jewish joke (and perhaps the best joke of all) is the definition of a Jewish telegram: "Start worrying. Details to follow." This is the state we're in a week after the American catastrophe. We don't know what's to come, we have no idea what trials await us. Evil is abroad in the world (and really, if the use of civilian passengers on a hijacked airliner as a bomb, knowing that they are not only going to die but to inflict a horrible death on thousands of others, isn't evil I don't know what is.) If ever there was a time to get serious, this is it, though as I write, we're not sure about what. There is much to fear. War. The chance of defeat. Under the Bin Laden Doctrine, every man is entitled to multiple wives. "Think of that," I suggested to a male friend. "Coming home every night to five Jewish wives. Five Jewish mothers. Five Jewish mothers-in-law." "What a horrible idea!" he cried.

It has been suggested that the reason for Jews having the best jokes in the world is that it is an evolutionary development which has grown up as a response to persecution. This is well conveyed in the story about the Jew in 1930s Germany who always reads Der Strummer, the Nazi propaganda sheet. In the Jewish press, he explains, everything is terrible: ghettoes, yellow stars, warnings of genocide. But in the Nazi paper it seems that Jews are all millionaire tycoons and financiers who run the world. As the assumption grows (erroneously, in the view of real experts on Bin Laden and his religious ideology) that the New York attack was a direct result of US complicity in the Israeli-Palestine conflict, I can report that WWIII, just like WWI and its blockbuster sequel WWII, is apparently all the fault of the Jews.

And, as if he had walked out from the pages of a Marvel comic, a new demon has been sent to torment us, an evil genius bent on world domination. I read recently, in Michael Chabon's novel The Amazing Adventures of Kavalier and Clay, that the comic-book superheroes were the creation of American Jews in the run-up to the second world war. Their message to themselves was: listen, we may look like weedy nerds with glasses, you might think you can push us around, but secretly we have Amazing Powers! Indeed we do. The fanatics reject our decadent culture and its lazy, selfish immorality. In its place they impose an intolerant ideology that expresses the purity and exclusive qualities of the religion or race. What they omit are the most resilient traits of the liberal west: its adaptability and humour, its capacity not to take bogeymen too seriously.

So whatever might be in store for us, this is not the moment to lose our ability to laugh. As early as Wednesday I started forwarding by email a link to a page on The Onion, the online newspaper satire. Searching its archive, I was certain that at some point I'd seen a spoof story about a suicide bomber, and I was right.

Here's a taste: "CHICAGO - His flight from O'Hare to LaGuardia delayed more than six hours, Hamas militant and would-be suicide bomber Nidal Hanani vowed never again to fly United Airlines. 'I do not have time for this,' said Hanani, seated at a Burger King in Concourse C, a plastic-explosives-filled duffel bag at his feet. 'My jihad against the west was supposed to be carried out shortly after takeoff at 8.35 this morning. It is now 2.50pm. How much longer must I sit around this airport like an idiot before God's will is done?' Compounding Hanani's frustration is the fact that his two previous flights - a commuter shuttle from Assira al-Shamaliyeh to Damascus followed by a redeye from Damascus to Chicago - were also delayed by more than four hours each."

On Sunday I received a phone call from my sister in Washington. According to the New York Times, to shore up the economy of the United States it was the patriotic duty of every American to go shopping as part of the war effort. With our credit cards between our teeth like commando knives, we should execute a series of daring raids on Bloomingdales and Bergdorf Goodman. I proposed to assemble a British expeditionary force to assist in this strategic manoeuvre. It's just a shame that Diana is not longer with us. She could have formed the Princess of Wales's Own Regiment, in which we would proudly serve.

Suicide bombers are said to die with smiles on their faces. They are smiling because they know that after they hit the detonator they will be with Allah in paradise. But the smile of the suicide bomber has as much to do with the source of true humour and laughter as the enforced rictus that accompanies the ritual incantation of "Have a nice day" in the supermarket checkout. Both are debased, forms of totalitarianism. The laughter I mean is not merely a release of tension but an act of defiance, a fist shaken at all rigid systems of order. Laughter disrupts, it disorders perceived patterns. In refusing to take life seriously, we're refusing to be enslaved. Its most fertile soil is a true perception and understanding of sadness, uncertainty and danger - not to belittle and trivialise it, but to overthrow it.

In the Manhattan tomb that used to be the World Trade Centre, there is the silence of the grave. Laughter is life. Laugh.

 Linda Grant won the 2000 Orange prize for fiction. Her third novel, Still Here, will be published by Little, Brown in May (chemical weapons, sarin gas attacks and nuclear war permitting).